1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to circuits utilizing current limiting.
2. Description of the Related Art
The use of voltage regulator circuits in electronic power supplies is very widespread. Voltage regulator circuits are used to provide a steady output voltage to the electronic circuit to which power is being supplied, typically referred to as the load. More particularly, the object of a voltage regulator circuit is to maintain a steady output voltage regardless of current drawn by the load.
FIG. 1 is a schematic diagram of one embodiment of a voltage regulator circuit. In the embodiment shown, an input voltage Vin is provided on the node coupled to the source terminals of p-channel transistors MP1 and MP2. The voltage regulator circuit provides an output voltage from the drain terminal of transistor MP2. Current through transistors MP1 and MP2 is controlled via a feedback path between the junction of resistors R1 and R2 (which comprise a voltage divider circuit) and the inverting input to the operational amplifier of the circuit. The operational amplifier is an error amplifier, used in the circuit to indicate an error between a reference voltage Vref (which is provided to the non-inverting terminal of the operational amplifier) and the voltage present at the junction of R1 and R2, i.e., the feedback voltage, or Vfb. The operational amplifier is configured to provide an output signal that is proportional to the difference between the reference voltage and the feedback voltage, which is used to drive the gate terminal of n-channel transistor MN3. This in turn controls the current I3 passing through transistor MN3. Since the drain terminal of transistor MN3 is coupled to the gate terminals of transistors MP1 and MP2, the value of I3 affects both of these transistors. The effect may be greater on transistor MP2, whose gate width N is typically greater than MP1, in some cases by several orders of magnitude.
One measure of the effectiveness of a voltage regulator circuit is its ability to respond to system transients. For example, if the load coupled to a voltage regulator is an integrated circuit (IC) in which a large number of drivers may switch states simultaneously, the demand for current from the voltage regulator may change suddenly. An ideal voltage regulator is able to meet the demand for increased current while maintaining its designed output voltage Vout. However, this may not always be practical for a given voltage regulator circuit and a given load. In practice, a load capacitance (coupled between the voltage output node and ground) is typically provided in order to meet the immediate demand for increased current. Using the circuit shown in FIG. 1 as an example, a load having a suddenly increased demand for current initially receives current from the load capacitance (not shown). However, the load capacitance can only provide a finite amount of current, after which the voltage regulator circuit must provide current for both the load as well as for recharging the load capacitance. When this occurs, the feedback voltage may be pulled down somewhat (assuming discharge of the load capacitance), thereby causing the amplitude of the error signal produced by the error amplifier to increase. This in turn results in an increased amount of current through transistors MN3 and MP2. Eventually, the increased amount of current through MP2 will cause both the output and feedback voltages to be pulled up through the voltage divider network. However, if this does not occur rapidly enough, damage to the voltage regulator circuit could occur, particularly if the load capacitance is significantly discharged. Even if there is no damage to the voltage regulator circuit, the inability to respond to the increased current demand may result in the output voltage remaining well below its intended value for a duration long enough to cause erroneous operation of the circuit(s) that make up the load as the load capacitance is recharged.
Modern electronic systems have placed increased demands on the operation of voltage regulator circuits. IC's having a large number of I/O pins (and thus a large number of drivers) can significantly change the current demand from a power supply system in an instant. Due to the high operational speed at which many IC's operate, voltage regulator circuits must be able to respond to this changing demand while maintaining an both an output voltage within a specified tolerance and the ability to recharge the load capacitance. This requires a voltage regulator circuit that responds quickly to transients. Furthermore, in some situations, a circuit used to implement a voltage regulator may be subject to short circuit or overload conditions for a significant amount of time. In such cases, the circuit may become damaged without protection against excessive currents that may result from such conditions. Similarly, other types of circuits (e.g., amplifiers) may also be susceptible to problems similar to those discussed above with regard to voltage regulators.